This invention relates to polymer encapsulated electrical components, and more particularly, to fibrous support layer reinforced electrical components, such as electrical resistance wires and integrated circuits, encapsulated in a polymeric layer.
Electrical resistance heating elements are used in many industrial, commercial and residential applications. They have been used to heat electroplating baths in the plating industry and can be found in the baseboard heaters of many homes. A typical construction for an electrical resistance heating element includes a pair of terminal pins brazed to the ends of a Nixe2x80x94Cr coil, which is then axially disposed through a U-shaped tubular metal sheath. The resistance coil is insulated from the metal sheath by a powdered ceramic material, usually magnesium oxide.
More modern heating elements have been developed using polymeric insulating materials around a resistance heating wire, such as disclosed in U.S. Pat. No. 5,586,214. These more recent devices employ resinous coatings which are often injection molded over the resistance wire. Since resistance wire is often extremely pliable, injection molding pressures are known to distort the circuit pattern for the wire in unacceptable ways. One solution described in the ""214 patent is to provide a polymer inner mold having a series of grooves for receiving the wire and holding it in place while a thermoplastic coating is injection molded over the assembly. This technique has been difficult to implement when thermoplastic materials are loaded with significant amounts of ceramic additives. Such mixtures are viscous and require great pressures of 10,000-25,000 psi to fill the mold properly. Even high mold pressures are sometimes insufficient to fill the details of the mold properly and the greater the mold pressure, the more stress is applied to the circuit pattern.
In still a further method described in U.S. Pat. No. 3,968,348, a resistance heating wire is disposed between a pair of fiberglass mats. A third fiberglass mat carries a heat dissipating metal foil. The three mats are separately impregnated with a thermosetting polyester resin and cured together to form a rigid, fluid impervious laminated structure. While laminating techniques have occasionally produced acceptable products, they often leave air gaps in the cross-section which make uniform heating difficult. Additionally, insufficient bonding to the glass mats or resistance wire can cause delamination, especially due to the difference in thermal expansion rates during heating and cooling cycles.
While such methods for creating resistance heating elements with thermoplastic or thermosetting polymers are known, there remains a need for better manufacturing processes which can further reduce the cost and improve the quality of polymer heaters. There also remains a need for more structural integrity during heating cycles and more effective thermal dissipation of heat from the resistance wire.
The present invention provides polymer coated electrical components and methods of fabricating such components, and is particularly useful in the manufacturing of electrical resistance heating elements. In one preferred embodiment of the invention, an electrical device is provided having a portion having a relatively thin cross-section, which is normally susceptible to distortion or dislocation by a viscous flow of a liquid or semi-liquid polymeric composition during an encapsulation of said electrical device by said polymeric composition. The electrical device is supported by a fibrous support layer disposed with the device. A polymeric layer is substantially encapsulated around the fibrous support layer and the electrical device.
It is known from experience that the application of polymeric material under pressure to delicate electrical devices can distort or move such devices in a mold. This invention employs a fibrous support layer to support electrical devices, such as thin gauge resistance heating wire, to provide structural support to the relatively thin cross-section of the electrical device so that a viscous polymeric composition can be used to encapsulate the device without distortion, displacement or loss of quality control.
The devices and methods of this invention enable relatively thin cross-sections as small as 0.01-0.3 cm to be encapsulated without distortion. The fibrous support layers of this invention help to maintain stability of viscous flows of thermoplastic or thermosetting material and act as a ballast or dampening factor. They also can be porous enough to allow the flow of polymeric material to penetrate through their pores so as to thoroughly coat the fibers and the electronic device. The flowing of polymeric materials through the fibrous support layers of this invention help to xe2x80x9cwetxe2x80x9d the fibers of these support layers to avoid a fracture initiation site along the boundary between the support layers and polymer, which has been known to cause part failure, especially when a part is heated.
The use of polymeric preforms in the manufacturing techniques of certain embodiments of this invention helps to more evenly distribute polymeric material over a greater amount of the device""s surface. The use of preforms and compression molding together, when compared to older injection molding techniques, distributes the molding forces more uniformly along the surface of the fibrous support layer to minimize movement of the electronic device during molding, and assist in the creation of a unified hermetic structure. Additionally, when thermosetting polymers are used, the use of the preform and fibrous support layers of this invention permits a lower compressive force to be used to liquefy thermosetting resins to permit them to flow properly. The preferred glass mats of this invention also improve the flexural modulus of the component by at least 50% over the unreinforced polymer used for encapsulation, and, preferably, have an air permeability rating of at least about 500 ft3/min.